Hearing aids
Last updated: April 14, 2010.
Hearing is one of our most important
senses because it alerts us
to dangers we can't always see. You might not notice a lorry
backing towards you, but if you hear the beep-beep-beep of a
reversing alarm you'll look up and get out of the way. Hearing
evolved in animals as an early-warning system, but for humans—the
most evolved animals of all—it's so much more than that. We
communicate largely by speaking and listening to one another and we
listen to music so we can experience deep emotions whenever we wish.
Imagine how frightening and isolating it can be, then, if you're
unlucky enough to be deaf from birth or if your hearing starts to
decline as you get older. Thankfully, science and technology—in the
shape of hearing aids—can help many people who suffer from hearing
problems. Let's take a closer look at hearing aids and find out how
they work.
Photo: A typical BTE (behind-the-ear) hearing aid—though half its components
sit inside the ear canal, as we explain below. Photo by Tyler W. Hill courtesy of U.S. Marine Corps and Defense Imagery.
How our ears hear sound energy
Sound is simply a kind of energy we can
hear. Things make sounds
when they vibrate (move back and forth), setting air in motion around
them. Think what happens when you bang a drum: the taut drum skin
vibrates very quickly, pushing and pulling on the air molecules (atoms joined together) that
are next to it. These air molecules move about more energetically and
start crashing into other air molecules too. That's how waves of
sound energy race out from a drum in all directions and that energy
(the same energy you gave to the drum skin by hitting it in the first
place) keeps travelling through the air until it reaches your ears.
Photo: Hearing begins with the outer ear, but
all the really clever apparatus that lets us sense and recognize sounds is
actually concealed inside our skulls. Why do we have strange folds in our outer ears? They help us
distinguish sounds coming from different directions.
What happens then? The pinnae (big outer
flaps) of your ears
are
shaped so they can gather sounds coming from different directions and
funnel them into the ear canal (the hole
that leads to your
inner
ear). At the end of your ear canal, there's a tiny drum-like skin
called the eardrum. When incoming sound
waves hit the eardrum,
they
make it vibrate. Three tiny bones called the hammer,
anvil,
and
stapes (or stirrup) in your skull detect
those eardrum
vibrations and
pass them on to a snail-shaped organ called the cochlea,
which
is
filled with fluid and tiny hairs called cilia.
The sound
vibrations
make the fluid in the cochlea wash back and forth, agitating the
cilia. The cilia detect those vibrations and send electrical signals
to your brain, which you hear as sounds of different frequency. In
short, then, hearing is all about sound energy entering your ears and
being turned into electrical impulses by tiny hairs inside your
cochlea.
Artwork: Anatomy of the human ear.
Picture by courtesy
of National Institute on Deafness and Other Communication Disorders (NIDCD) and
National Institutes of Health Photo Galleries.
How we can lose our hearing
The path between your outer ear and your brain can be blocked or
damaged in many different places and in a number of different ways,
so people can become deaf or lose some or all of their hearing for
lots of different reasons. One of the most common types of hearing
loss happens when the hairs in the cochlea become damaged. If there
are fewer hairs, sounds produce less stimulation in your brain—so
things need to be louder for you to hear them. That's where hearing
aids come in. They can't help everyone with impaired hearing, but
they can often make a difference to hearing problems caused by a loss
of cochlear hair cells (medically known as sensorineural hearing
loss).
How hearing aids work
There's a tired old joke in television sit-coms where people shout
at a
deaf person (usually a deaf, elderly person) to make themselves
heard. What happens if you shout at a deaf person is that you
transmit sound waves of greater amplitude (volume) and energy into
their ear
canal. Their cochlear hair cells are more likely to detect these more
energetic sound waves and, consequently, they're more likely to hear
you. Old-style ear trumpets work a slightly different way.
Effectively, they make the outer ear much bigger and concentrate the
energy in incoming sounds into a smaller area. That increases the
pressure that sounds make on the eardrum and, again, improves the
person's chances of hearing.
While shouting louder and using ear trumpets are crude, mechanical
solutions to the problem of hearing loss, a hearing aid is a much
more sophisticated electrical solution. A
hearing aid is
simply an electronic sound amplifier.
You've seen people on stage
speak into a microphone and have their
voices hugely amplified by
giant loudspeakers so crowds can hear
them? A hearing aid works
exactly the same way, except that the microphone, amplifier, and
loudspeaker (and the battery that powers
them) are built into a
small, discreet, plastic package worn
behind the ear or just inside
the ear canal.
Photo: Right: A BTE (behind-the-ear) hearing aid. You can clearly see the pink-colored case that sits behind the ear and the clear plastic tube leading to the ear mold at the bottom. Photo by Tyler W. Hill courtesy of U.S. Marine Corps and Defense Imagery.
One of the most common types of hearing aid is called a BTE
(behind the ear) and consists of two separate pieces. Behind
the
ear,
there's a hard plastic case that contains a small microphone,
amplifier, and loudspeaker. This is linked, via a tube, to a softer
plug called an ear mold shaped to fit just
into the person's
ear
canal. When you wear a hearing aid like this, the microphone picks up
sounds around you and turns them into an electric current, the
amplifier (using one or more transistors)
boosts the size of the
current, and the loudspeaker turns the boosted current back into a
much louder sound. This amplified sound flows through the tube and
the ear mold into the person's ear. A different style of hearing aid
called an ITE (in the ear) has all the same
components but
fitted
into a small plug that pushes into the ear canal. ITE hearing aids
are so discreet that you may not even notice someone is wearing one,
especially if they comb their hair forward to cover their ears.
Photo: Left: Hearing aids as they used to be. This Acousticon aid dates from 1925. You wore the top, headphone part over your ear. The bottom part contained the microphone, battery, and a control (left) for adjusting the volume. This is an exhibit at Think Tank, the science museum in Birmingham, England.
Hearing aids come in two main kinds. Analog
hearing aids simply convert sound into electric currents, boost the currents, and turn
them back into louder sounds. Digital
hearing aids are more
sophisticated (and cost much more). They convert the sound into a
numerically coded signal and, depending on how they are designed,
process and refine the signal before turning it back into a sound.
Digital hearing aids can be tuned so they emphasize sounds of
particular frequency or block out unwanted noise more effectively,
wheras analog hearing aids tend to amplify everything (background
noises as much as important sounds) by the same amount.
Although a hearing aid can never restore hearing completely, it
can make a huge difference to a person's life by helping them
converse more normally and enjoy everything from TV and radio to
recorded music and birdsong. It's a great example of how science and
technology (often much maligned) can really improve the quality of
our everyday lives!
